A number of different applications require precise control of light energy at relatively high levels in order to effect a change, such as initiating a chemical reaction or state change. In micro lithography apparatus and systems, widely used in the fabrication of microcircuits of various types, an intense beam of actinic light, suitably shaped and conditioned by a complex and costly optical system, is projected through a projection lens and transmitted through a patterned mask and onto a planar wafer of semiconductor material. This material has been preconditioned with a photoresist material to respond to the actinic light and thereby form a corresponding pattern that provides a portion of the circuitry. The needed light energy levels are typically provided by an ultraviolet (UV) light source, emitting actinic light at below 400 nm.
Conventional microlithography systems have relied on arc lamps and related types of gas discharge lamps as their UV sources. Due to their favorable brightness levels and overall efficiency, arc lamps have been successfully utilized for microcircuit fabrication, offering good resolution above about 1 micron or more. However, arc lamps have a number of unfavorable characteristics, including the following:                (i) Requirement for filtering out the non-actinic wavelengths that are emitted. The arc lamp emits a range of wavelengths in addition to the (actinic) UV light that is used for lithography, such as I-line UV light at 365 nm. The need to filter out the unused light has the unavoidable effect of attenuating some portion of the UV light at the same time.        (ii) High power requirements. Because the useful light for microlithography is only a small percentage of the optical power emitted, these lamps have significant power consumption. The required electrical power can exceed the useful optical power by as much as two orders of magnitude.        (iii) Catastrophic failure modes. Due to the high pressures internal to the lamp structure, failure of the arc lamp generally results in an explosion, with the potential to damage neighboring optical components. Lost time and cost result from the needed cleanup and repair.        (iv) Hazardous substances. Arc-lamps used for UV radiation typically contain hazardous substances, such as mercury. These substances burden the end-user for handling, storage, and disposal of light sources.        (v) Limited component lifetime. Arc lamps have a relatively short useful life, requiring replacement every 700 to 1000 hours.        
Added to these disadvantages is the characteristically non-uniform illumination profile for arc lamp emission. To a first approximation, the arc lamp gap is considered to provide a point source, radiating light equally in all directions. A curved reflector gathers and redirects this light for use by the lithography optical system.
For film projectors and other conventional devices, considering the arc lamp as essentially a point source may be sufficient. However, this characterization of the arc lamp is inaccurate for microlithography, which demands extremely high resolution and uniformity. For demanding applications of this type, the illumination energy emitted from the arc lamp and curved reflector, considered in the spatial and angular domains, falls short of what would be provided by a perfect point source. This is noted, for example, in U.S. Patent Application Publication No. 2005/0231958 entitled “Illumination System with Improved Optical Efficiency” by Cutler et al. In particular, the Cutler et al. disclosure describes the “donut hole” intensity profile that inadvertently results from blocking some of the light reimaged onto the source itself from the parabolic reflector. To compensate for this “intrinsic” property of re-imaged arc lamp emissions, Cutler et al. propose a number of improvements to conventional geometries for reflector shape and positioning with arc lamps.
Conventional systems solutions compensate for non-uniformity and other inherent drawbacks of the arc lamp further along the optical path. For example, U.S. Pat. No. 5,675,401 entitled “Illuminating Arrangement Including a Zoom Objective Incorporating Two Axions” to Wangler et al. notes the problem of the dark spot in an arc lamp and compensates using two axions in the optical path to collapse the beam. Other solutions adjust various other optical path components in order to provide a more uniform beam intensity distribution or, alternately, to adapt the inherent beam intensity distribution to achieve uniformity or some particular desired arrangement, such as using a neutral density (ND) filter or wedge in the path of illumination. In a number of cases, other optical components are used to adapt the uniformity of illuminance. For example, one type of approach employs arrays of lenses to alter the light distribution of an optical system by superposition of source illumination. In another approach, an apodizing aperture is provided in cooperation with a cylindrical lens for forming a uniform line of illumination using an arc lamp source.
Another approach used for controlling the intensity profile employs a spatial light modulator, such as a micromirror array, in the path of the source illumination. This strategy is described, for example, in U.S. Pat. No. 7,133,118 entitled “Lithographic Apparatus and Device Manufacturing Method” to Gui et al. This provides some capability for shaping the distribution profile of light provided, but at the cost of considerable added complexity and additional components. Other approaches attempt to adapt the cross-sectional shape of the beam to achieve an appropriate “illumination geometry”, as described in U.S. Pat. No. 6,233,039 entitled “Optical Illumination System and Associated Exposure Apparatus” to Yen et al.
In general, it is costly and time-consuming to adapt and adjust the design of system optics in order to compensate for inherent non-uniformity of the light source. This becomes increasingly difficult for higher power microlithography systems, particularly where precision requirements become more stringent. Further, the optics themselves can be fairly complex and require laborious procedures for adjustment and alignment to compensate for imperfections in lens shape and mount precision. These inherent difficulties can also work against achieving a desired illumination profile, since it is sometimes necessary to compromise between making different types of adjustment in order to achieve suitable optical performance.
In microlithography, as in other high-power light applications, conventional solutions add the burden of adjustment for compensating problems with light source uniformity to the already difficult task of adjustment and alignment of the complex optical system overall. It would be advantageous to provide an illumination system that not only reduces the need for adjustment of downstream optics to compensate for uniformity and telecentricity problems, but can even provide a mechanism for adapting the illumination profile to compensate for imperfections in system optics or to enhance other process variables in microlithography fabrication applications.